I agree with Dwight and Todd - a Wafer Level CSP (WLCSP) is really a flip
chip device (silly marketing definitions - just causes confusion!) so
without the application of an underfill material, there are going to be
solder joint cracks/failures unless the product use environment is really
benign.

Dave

On Wed, Apr 20, 2016 at 9:17 AM, Mattix, Dwight <[log in to unmask]>
wrote:

> S/M define pads on a ball and no underfill or corner staking?   Standby
> for cracked solder joints.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of MacFadden, Todd
> Sent: Wednesday, April 20, 2016 5:10 AM
> To: [log in to unmask]
> Subject: Re: [TN] .031 PCB and WLCSP
>
> Without underfill in this application your greatest challenge for solder
> joint reliability will likely be from drop rather than cyclic strain and
> fatigue. It's a small device (9x9mm) so that risk may be relatively low,
> but here are some things you may consider:
>
> * PCB-side solder joint area should match the device side pad size area
> (i.e., UBM). The UBM diameter will be smaller than the 0.2mm ball diameter
> (you may need to ask the device supplier for this info since it's not
> usually provided on the datasheet).
>
> * Use solder mask defined pads because: (1) a Cu-defined PCB pad <0.2mm is
> not possible by most PCB fabricators if there are uvias in the pads; (2)
> solder area of the PCB pads should be of consistent size; this is not
> possible with Cu-defined pads due to exit traces, which draw solder away
> and distort the shape of solder joints inconsistently. Solder mask defined
> also allows for larger Cu pad, for which the PCB fabricator will thank you.
>
> * Is corner staking an option? In the absence of full underfill, your best
> bet to pass drop test may be to apply epoxy dots or lines on the corners.
>
> Good luck!
> Todd
>
>
> -------------------------------------------------------
> Todd MacFadden
> Component Reliability Engineering
> Bose Corporation
> 1 New York Ave, MS 415
> Framingham, MA 01701
> 508.766.6259
> -------------------------------------------------------
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Curt McNamara
> Sent: Tuesday, April 19, 2016 6:56 PM
> To: [log in to unmask]
> Subject: [TN] .031 PCB and WLCSP
>
> I am reviewing a design with a .4 mm pitch, .2 mm ball WLCSP (9x9) on a 4
> layer .031 FR4 pcb.
>
> These will be used like a remote control, so there will be force applied,
> however there are supports for the PCB.
>
> Due to the presence of switches with cleaning restrictions, underfill is
> not possible.
>
> Looking for any comments on potential reliability concerns. The design
> could be changed to .064 if that would help.
>
> Thanks in advance!
>
>     Curt
>
>
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